Quantification of Low-Frequency Stoneley Wave Energy Attenuation in a Carbonate Fractured Bedrock Aquifer: An Evaluation of Its Relationship with Fracture Hydraulic Properties

Resolving hydrogeologic conditions in fractured bedrock aquifers poses unique challenges due to heterogeneity attributed to variability in fracture network characteristics, where fractures, if open and connected, serve as primary pathways for water flow and contaminant transport. Bedrock mechanical properties influence fracture frequency and connectivity, and thereby hydrologic unit (HGU) boundaries and hydraulic conductivity variations. In such aquifers, HGU boundaries are often associated with zones of poor vertical fracture connectivity, caused by the termination of vertical joints at changes in rock mechanical properties associated with bedding. Understanding these properties and boundaries is essential for improved hydrogeologic conceptual site models relied on for science-based groundwater management and source protection.  

This study focuses on downhole data collected in one cored hole within a regionally significant dolostone aquifer in the City of Guelph, Ontario, Canada.  Full Waveform Sonic (FWS) low-frequency energy attenuation, combined with other advanced borehole geophysical and hydraulic datasets, is used to investigate the relationship between insitu fracture characteristics and hydraulic properties for 9 depth-discrete intervals.    

The literature reports that attenuation of the Stoneley wave’s low-frequency energy can be diagnostic of wave-induced fluid motion and viscous dissipation associated with hydraulically open fractures; however, quantitative borehole-scale estimations remain scarce. We compare low-frequency energy attenuation under continuous (slow logging speeds of 0.2-0.3 m/min) acquisition with other complementary high-resolution borehole datasets to assess the relationship between fractures’ hydraulic properties and energy attenuation. The other datasets include high-resolution temperature, Nuclear Magnetic Resonance (NMR), as well as depth-discrete transmissivity from straddle packer testing and hydraulic head profiles from numerous, temporarily deployed pressure transducers sealed behind flexible fabric borehole liners. By linking fracture hydraulic properties and energy attenuation, the study aims to improve distinguishing hydraulically active fractures during borehole characterization to better understand fracture controls on aquifer and aquitard unit boundaries and contaminant transport in fractured sedimentary rocks.